On the Chief Line in the Spectrum of the Nebulas. 175 



spectrum of nitrogen to permit this line to be used as a fiducial line 

 of comparison. The line in the spectrum of the nebulae is narrow 

 and defined, while the line of nitrogen is double, and each component 

 is nebulous and broader than the line of the nebulae. The nebular 

 line is apparently coincident with the middle of the less refrangible 

 line of the double line of nitrogen* ... In the course of some 

 other experiments, my attention was directed to a line in the spectrum 

 of lead which falls upon the less refrangible of the components of the 

 double line of nitrogen. The line appears to meet the requirements 

 of the case, as it is narrow, of a width corresponding to the slit, 

 defined at both edges, and in the position in the spectrum of the 

 brightest of the lines of the nebulae. 



" In December, 1872, I compared this line directly with the first 

 line in the spectrum of the Great Nebula in Orion. I was delighted 

 to find the line sufficiently coincident in position to serve as a fiducial 

 line of comparison. 



" I am not prepared to say that the coincidence is perfect ; on the 

 contrary, I believe that, if greater prism power could be brought to 

 bear upon the nebulae, the line in the lead spectrum would be found 

 to be in a small degree more refrangible than the line in the 

 nebulae. 



" The spectroscope employed in these observations contains two 

 compound prisms, each giving a dispersion of 9 6' from A to H. A 

 magnifying power of sixteen diameters was used. 



" In the simultaneous observation of the two lines it was found that, 

 if the lead line was made rather less bright than the nebular line, 

 the small excess of apparent breadth of this latter line, from its 

 greater brightness, appeared to overlap the lead line to a very small 

 amount on its less refrangible side, so that the more refrangible sides 

 of the two lines appeared to be in a straight line across the spectrum. 

 This line could be therefore conveniently employed as a fiducial line 

 in the observations I had in view." 



1877. The measures, by Dr. Copeland and Lord Lindsay, of the 

 wave-length of the line near 500 in Nova Cygni, which has generally 

 been accepted as the nebula line, were aa follow : t 



1877, January 2 502'4 



505-1 



8 502-9 



9 500-7 



27 5008 



Mean 502 '4 



' Roy. Soc. Proc.,' vol. 20, p. 380. 

 ' Copernicus,' vol 2, p. 101. 



